Rosuvastatin calcium has the chemical name (7-[4-(4-fluorophenyl)-6-isopropyl-2-(N-methyl-N-methylsulfonylamino)pyrimidin-5-yl]-(3R,5S)-dihydroxy-(E)-6-heptenoic acid-calcium salt), and has the following chemical formula:
Rosuvastatin calcium is an HMG-CoA reductase inhibitor, developed by Shionogi for the once daily oral treatment of hyperlipidaemia (Ann Rep, Shionogi, 1996; Direct communications, Shionogi, 8 Feb. 1999 & 25 Feb. 2000). Rosuvastatin calcium is a superstatin, which can lower LDL-cholesterol and triglycerides more effectively than first generation statin drugs.
Rosuvastatin calcium is marketed under the name CRESTOR for the treatment of a mammal such as a human. According to the maker of CRESTOR, it is administered in a daily dose of from about 5 mg to about 40 mg.
U.S. RE Pat. No. 37,314 discloses the preparation of Rosuvastatin calcium, wherein the step of removing the alcohol protecting group, R2, of the intermediate 1
to obtain the intermediate 2
is performed by using a solution of hydrofluoric acid. However, the use of hydrofluoric acid is problematic on an industrial scale because of the strong corrosive properties and very toxic vapors; contact with glass or metal should also be avoided.
An alternative method for removing the silyl protecting group of the intermediate 1 is disclosed in U.S. patent Ser. No. 05/222,415. According to the disclosure of this application, methanesulfonic acid in methanol is used instead of hydrofluoric acid; however, this process may lead to contamination of the final product by the impurity rosuvastatin-calcium-methylether, as exemplified in example 4.

Rosuvastatin calcium, like any synthetic compound, can contain extraneous compounds or impurities originating from various sources. These impurities in rosuvastatin calcium, or any active pharmaceutical ingredient (API), are undesirable and, in extreme cases, may even be harmful to a patient being treated with a dosage form containing the API.
Impurities in an API may arise from degradation of the API itself, which is related to the stability of the pure API during storage, and from the manufacturing process, including the chemical synthesis of the API. Process impurities include unreacted starting materials, chemical derivatives of impurities contained in starting materials, synthetic by-products of the reaction, and degradation products.
The stability of an API during storage is a critical factor in the shelf life of the API, and so affects the ability to commercialize an API. The purity of the API resulting from the manufacturing process also affects the ability to commercialize an API. Impurities introduced during commercial manufacturing processes must be limited to very small amounts, and are preferably substantially absent. For example, the ICH Q7A guidance for API manufacturers requires that process impurities be maintained below set limits by specifying the quality of raw materials, controlling process parameters, such as temperature, pressure, time, and stoichiometric ratios, and including purification steps, such as crystallization, distillation, and liquid-liquid extraction, in the manufacturing process.
At certain stages during processing of an API, it must be analyzed for purity because the product of a chemical reaction is rarely a single compound with sufficient purity to comply with pharmaceutical standards. Side products and by-products of the reaction and adjunct reagents used in the reaction will, in most cases, also be present in the product mixture. Typically, the API is analyzed by HPLC or TLC analysis to determine if it is suitable for continued processing and, ultimately, for use in a pharmaceutical product. The API need not be absolutely pure, as absolute purity is a theoretical ideal that is typically unattainable. Rather, purity standards are set with the intention of ensuring that an API is as free of impurities as possible, and thus, is as safe as possible for clinical use. As discussed above, in the United States, the Food and Drug Administration guidelines recommend that the amounts of some impurities be limited to less than 0.1 percent.
Generally, side products, by-products, and adjunct reagents (collectively “impurities”) are identified spectroscopically and/or with another physical method, and then associated with a peak position, such as that in a chromatogram, or a spot on a TLC plate. (Strobel p. 953, Strobel, H. A.; Heineman, W. R., Chemical Instrumentation: A Systematic Approach, 3rd dd. (Wiley & Sons: New York 1989)). Thereafter, the impurity can be identified, e.g., by its relative position on the TLC plate (wherein the position on the plate is measured in cm from the base line of the plate) or by its relative position in the chromatogram of the HPLC (where the position in a chromatogram is conventionally measured in minutes between injection of the sample on the column and elution of the particular component through the detector). The relative position in the chromatogram is known as the “retention time.”
The retention time can vary about a mean value based upon the condition of the instrumentation, as well as many other factors. To mitigate the effects such variations have upon accurate identification of an impurity, practitioners use the “relative retention time” (“RRT”) to identify impurities. (Strobel p. 922). The RRT of an impurity is its retention time divided by the retention time of a reference marker or reference standard. It may be advantageous to select a compound other than the API that is added to, or present in, the mixture in an amount sufficiently large to be detectable and sufficiently low as not to saturate the column, and to use that compound as the reference marker or reference standard for determination of the RRT.
As is known by those skilled in the art, the management of process impurities is greatly enhanced by understanding their chemical structures and synthetic pathways, and by identifying the parameters that influence the amount of impurities in the final product.
In this application the impurity rosuvastatin calcium-alkylether in the API is used as the reference marker or reference standard.
There is a need in the art for rosuvastatin calcium having low levels of rosuvastatin-calcium-alkylether, and for processes of preparing rosuvastatin calcium having a lower level of rosuvastatin-calcium-methylether.